Method of preparing cellulosic paper containing acrylamide copolymer and product thereof



United States Patent 3,255,072 METHOD OF PREPARING CELLULOSIC PAPERCONTAINING ACRYLAMIDE COPOLYMER AND I PRODUCT THEREOF David P. Sheetzand Charles G. Humiston, Midland, Mich, assignors to The Dow ChemicalCompany, Midland, Mich. a corporation of Delaware No Drawing. Filed July29, 1963, Ser. No. 298,435 6 Claims. (Cl. 162168) This invention isconcerned with cellulosic webs having improved wet and dry strengths andis particularly directed to paper having incorporated therein a smallbut effective amount of a polymer containing pluralities of primaryamino and amide groups and to a method for the production of paper sotreated.

In the past, various materials have been applied to fibrous products,especially to aqueous dispersions of cellulosic fibers prior to theformation of felted webs therefrom, in order to improve the dry strengththereof. For example, Canadian Patent 477,265 describes the use ofhydrolyzed polyacrylamide to improve the dry strength of paper.Optimization in the use of this material is highly dependent upon thepresence of controlled amounts of alum. Similarly, naturally occurringmaterials such as starch and locust beam gum produce moderate drystrength improvements when employed conjunctively with'alum. It would bedesirable to have available a dry strength additive which could beemployed independently of alum if such were desired. This might be thecase as, for example, when preparing paper from a neutral or alkalinestock.

Other dry strength additives of the prior art include those of UnitedStates Patent 2,884,057 wherein the use of poly-quaternary ammoniumcompounds is suggested for producing paper of improved dry strength. Itis characeteristic of the latter additives that they have little, ifany, effect on the wet strength of the paper.

Another patent of interest in connection herewith is United StatesPatent 2,838,397. This patent describes the use of certain nitrogenouspolymers as :filler rentention aids in the manufacture of paper. As willbe elaborated more fully hereinafter the practice of these teachingsinterferes with and substantially diminishes the results obtained withthe unique paper additives of the present invention.

It would be most desirable and it is an object of the present inventionto provide a water-soluble, and thus easily applied, agent to enhanceboth the wet and dry strengths of paper. Particularly, it is an objectto provide a dry strength additive of which small amounts impart to wetfelted cellulosic products superior improvements in dry strength andmoderate wet strength properties, i.e., the latter being such thatrepulping or broke recovery problems are avoided. A further object is toprovide a polymeric strength additive for paper which does not requirefor its effective use the presence of alum in the furnish. Other objectsand benefits will become apparent from the following specification andclaims.

In accordance with the present invention, it has been discovered thatenhanced dry and wet strengths are produced in paper by incorporating incellulosic slurries employed to prepare the paper a small proportion ofa watersoluble polymer containing intralinear units of the generalformulae:

a i Tit TY In the above formulae, R and R are independently selectedfrom the group consisting of methyl and hydrogen, n is an integer from 2to 3 inclusive and x and y are the number of monomer units in thepolymer. The x/y ratio, or mole ratio of Formula A to Formula B, in thefinished polymer can vary from about 50 to about 1.5. In a preferredcopolymer of acrylamide and 2- aminoethyl methacrylate, this range ofmole ratios corresponds to interpolymers of from about 97 to about 45Weight percent of acrylamide and correspondingly from about 3 to about55 weight percent of Z-aminoethyl methacrylate. While the describedpolymers can be employed as the free base, the mineral acid salts ofsuch polymers are preferred. The amount of the polymeric additive usedis that amount suificient to produce a significant increase in the drystrength properties of the paper.

A'unique feature of the above polymers is their performance without thepresence of alum. Although the presence of alum does not detract fromthe effectiveness of the above polymers, its presence is definitely notrequired for superior results. An additional unique feature of the abovepolymers resides in the fact they perform as intended in a substantiallyimproved fashion in the absence of inorganic paper fillers. Thus, formaximum dry strengths in accordance with the invention the polymersshould be utilized with cellulosic pulps substantially free of filtermaterials, e.'g., calcium carbonate, clay and pigments.

Fundamentally, the polymers eifective in the invention are water-solublecopolymers derived by polymerization through the ethylenic double bondof at least one monomer selected from primary aminoalkyl esters ofacrylic and methacrylic acids wherein the alkyl group contains 2 or 3carbon atoms, and mineral acid salts of such esters with at least onemonomer of the group of acrylamide and methacrylamide.

The effective polymers may also contain small amounts, e.g., up to about25 percent by weight of the total polymer, of other suitablemonoethylenically unsaturated acrylic monomers having up to 7 carbons.Examples of such optional monomers are N-methylolacrylamide,acrylonitrile, methacrylonitrile, ethyl m-hydroxymethylolacrylate andalkyl and hydroxyalkyl esters of acrylic and'methacrylic acids whereinthe alkyl group contains from 1 to 3 carbon atoms. While they are notnecessary for effective results, the presence of the optional acrylicmonomers in the interpolymer does not detract from its generalperformance capabilities for the purposes of the inven-tion.

For use in accordance with the invention, the aminoalkyl and amideacrylic copolymer should be of moderately high molecular weight, thatis, such copolymer should have a Staudinger molecular weight of at leastabout 5,000. Above this limit, increasing molecular weights, whilegiving generally better results, have a comparatively small effect onthe strength properties of the finished paper as compared to themagnitude of the overall strength improvement. In practice, the relativeviscosity of a solution of such polymers dissolved in 2 percent aqueoussodium chloride is a convenient criterion of the polymer molecularweight. Good results have been obtained when employing polymer agentscharacterized by relative viscosities from as little as 1.2 up to asmuch as 9 or more, the latter limit corresponding to an approximateStaudinger molecular weight of about 1,000,- 000. The relative viscosityis the viscosity of an aqueous solution containing 0.5 percent by weightof the polymer and 2 percent by weight sodium chloride divided by theviscosity of the 2 percent sodium chloride solution, the viscositymeasurements being made at 25 C. with; a Ubbelohde viscosimeter.

The aminoalkyl ester monomers described herein are unstable in alkalinesolutions and are, therefore, usually polymerized in the form of theirmineral acid salts. Suitable mineral acids include hydrochloric,hydrobromic, sulfuric, nitric, phosphoric and the like acids. Generally,any mineral acid can be used that has a pK below about 4. The copolymersare readily prepared by carrying out the polymerization of monomers ofeach Formula A and Formula B in an aqueous solution of the monomers,employing a water-dispersible catalyst, e.g., 'an organic or inorganicperoxide, capable of yielding free radicals. When it is desired tointroduce a minor proportion of an optional water-insoluble comonomer,such as an alkyl methacrylate, polymerization may be effected in anorganic solvent system such as a lower alkanol. Polymerization modifierssuch'as thiourea, Z-mercaptoethanol or the like, can be incorporated inthe polymerization recipe if desired. Also, the concentration ofcatalytic metal cations, if any, in the reaction mixture can becontrolled by suitable additions of a chelating agent such as a salt-ofethylenediaminetetracetic acid or of diethylenetriarnjinepentaceticacid. A preferred catalyst system consists of a small percentage ofhydrogen peroxide and thiourea together with a few parts per million ofcopper ion.

A recipe for preparation of a preferred copolymer of the invention is asfollows:

Ingredient: Percent by weight 2-aminoethy1 methacrylate hydrochloride 5Acrylamide 20 Thiourea 0.2 Cupric ion p.p.m. 5 Hydrogen peroxide 0.4Water, to make 100 P.p.m. indicates parts by weight per million parts oftotal charge.

tain from as little as 0.01 percent up to much as percent or more byweight polymer solids. Generally, the dilute solution of the copolymeris added with suitable mixing to paper pulp at any point'prior toformation of the paper web. While it may be added prior to com- Ipletion of the beating and/ or refining steps, it is preferred polymerin the form of a viscous aqueous solution there- In large scalereactions, cooling of the reaction mixture may be required to avoidrunaway reactions. In general, it is desirable to regulate the rate ofreaction so that a peak temperature in the range of from 80 to 100 C isreached. Alternatively, the reaction rate may be controlled byintroducing an aqueous solution of the monomers gradually or portionwiseinto a reaction vessel containing a solution of the polymerizationcatalyst. Oxygen appears to have an inhibitory effect on thepolymerization reaction so that for rapid initiation of polymerization,it is desirable to purge the reaction mixture with an inert gas such asnitrogen prior to addition of the hydrogen peroxide. Although a dryproduct can be recovered by conventional separatory processes, e.g.,drying on rolls or precipitation in solution with a water-misciblenon-solvent for the polymer such as methanol, the copolymers prepared inthe above manner are preferably maintained in aqueous solution formuntil used.

In carrying out .the invention, an aqueous stock solution of thecopolymer is mixed with sufficient water to prepare a dilute solution,e.g., such solutions usually conto incorporate the copolymer into aturbulent stream of the pulp after the latter two operations. For bestresults the pH of the treated fiber suspension is maintained within therange from about 4 to about 10. In addition to the foregoing modes ofoperation, it is also possible to incorporate the polymeric additamentinto a preformed paper web as by spraying it with or dipping it in asolution of the copolymer and thereafter drying the coated a er.

p Vhen the polymeric addit-ament of the invention is added to the paperpulp prior to formation of the paper web, it is desirable, but notessential for good results, that it be added before completion of theaddition of other paper-making ingredients. For optimum results,however, it is essential to avoid the use of fillers which have adeleterious effect on the dry strength of the finished paper product.Further, while alum can be used with the copolymers of the invention, itis not required for good performance. If alum is employed, it may beadded either before or after the addition of the copolymer.

In one conventional mode of operation, the copolymer solution isintroduced at the inlet side of the fan pump on a conventionalpapersheeting machine, such as a Fourdrinier. Alternatively, thecopolymer solution may be mixed with the pulp suspension as it entersthe headbox, provided there is sufficient agitation in the headboxassembly to provide lfOI thorough mixing of the polymer with the pulpprior to formation of the paper web. A sufiicient amount of thecopolymer solution is employed to provide from about 0.01 to about 3percent, preferably from about 0.2-5 to about 1.5 percent, by weight ofthe polymer, based on the weight of cellulosic fiber in the pulpsuspension.

Fol-lowing the addition of the copolymer, the treated pulp suspensionmay be employed for the preparation of paper in conventional fashion,as, for example, by the use of handscreens or by modern, high-speedpaper-making machinery. The paper web so formed may be pressed and driedin conventional fashion.

The following examples illustrate the invention but arenot to beconstrued as limiting the same.

Example I 360 grams of unbleached, air-dried soda pulp from hardwoodswas distributed in 23 liters of water in a Valley laboratory beater andcirculated until thoroughly dispersed. Thereupon a 5,500 gram load wasplaced on the beater bedplate arm and the pulp was beaten to a freeness(Canadian Standard) of about 400. 325 milliliter portions of the beatenpulp suspension were diluted with 2 liters of water to produce a furnishfor test handsheets. In each determination, at least one such portion ofthe suspension was employed without further treatment to prepare a handsheet as an untreated check. When treated paper was prepared, asuflicient amount of one of the aminoethyl acrylate and acry-lamidecopolymers (hereinafter called lgenerically amino-amido acryliccopolymers) was added as an aqueous solution to the beaten and dilutedfiber suspension to provide the desired proportion of copolymer tocellulosic fiber on a dry basis. Handsheets were made from the treatedand untreated furnishes essentially in accordance with TAPPI StandardMethod T205 m-53 using a Williams 10 inch by 12 inch mold modified toemploy a sample size as specified above. After the [furnish had drainedthrough the wire of the h-andsheet mold to form a wet paper sheet, thewet sheet on the screen was placed between blottcrs and thereafter thecombination was placed in a 12 inch by 12 inch hydraulic press .andpressed lightly until the wet sheet adhered to the adjacent blotter. Thescreen was then removed and replaced with a polished metal plate and thelatter was backed with additional blotters. The resultant stack was thenplaced in the hydraulic press and compressed for 3 minutes at a pressureof 60 pounds per square inch. This procedure caused the moist sheet toadhere to the metal plate. The blotters were then separated from theplate with adherent sheet and the latter was clamped in a frame anddried in a forced air oven at 110 C. for 15 to 30 minutes. Thereafter,the dried handsheet was separated from the plate and conditioned in achamber maintained at 70 CF. and 62 percent relative humidity for atleast 48 hours before testing for strength properties.

Following the foregoing procedure, a series of handsheets was preparedwithout additaments and dried for 30 minutes at 110 C. to serve asuntreated checks. In similar manner, another series of handsheets wasprepared, each sheet containing one percent, by weight of the drycellulosic fiber, of one polymer of a series consisting of copolyrnersof Zaminoethyl methacrylate hydrochloride with varying proportions ofacrylamide. A homopolymer of 2-aminoethyl methacrylate was also run forpurposes of comparison. All the resulting handsheets were conditioned asdescribed above and tested for tensile strength, dry bursting strengthand basis weight according to TAPPI Standard Method T220 m-5 3. Wetburst strengths were determined by TAPPI Standard Method after soakingtest portions of handsheets in deionized water for 24 hours. All testresults were corrected to a constant basis weight of 50 pounds. Theresults are summar'ued in Table 1 wherein the [figures in the Treatmentcolumn From the foregoing it will be observed that the copolymerproduces superior dry strength coupled with a moderate increase in wetstrength. Only in this latter regard do any of the amino-amide acryliccopolymers fall below the performance level of the homopolymer of AEM.The wet strength improvement of the copolymer, particularly incomparison to the wet strength performance levels achieved with thehomopolymer, when up to nearly 95 weight percent acrylamide isincorporated into the copolymer is surprising in view of the fact thatpolyacrylamide itself gives essentially no wet. or dry strengthimprovement under these test conditions, i.e., in the absence of alum.

Example 2 In a manner similar to the procedure of Example 1, severaltest handsheets were prepared in which one percent by weight of apolymeric additive was incorporated into the finished paper. Thepolymers added included a co polymer of the invention containing 80weight percent acrylamide and 20 weight percent Z-aminoethylmethacrylate hydrochloride and for the purposes of comparison polyN,N-dimethyl2-aminoethyl methacrylate hydrochloride,poly-N-methyl-Z-aminoethyl methacrylate hydrochloride, poIy-Z-aminoethylmethacrylate hydrochloride and polyacrylamide.

After drying at 110 C., the handsheets were placed in a constanttemperature and humidity chamber at F. and 62% relative humidity. Thesheets were thus conditioned for at least 48 hours before beingsubjected to tests to determine their strength properties according toTAPPI Standard T220 m-53. Results of these tests are tabulated below inTable 2 according to the prior polymer treatment of the handsheet.

refer to the proportions by weight of the indicated monomers combined inthe copolymer employed and the values indicated for strength propertymeasurements are averages of several determinations. [For example, 95/5AA/AEM represents a copolymer of 95 parts by weight of acrylamide with 5parts by weight of 2-aminoethyl methacrylate hydrochloride and 0/100AA/AEM represents the homopolymer of Z-aminoethyl methacrylatehydrochloride.

From the foregoing it will be observed that the copolymer of theinvention provided excellent improvement in the dry and tensilestrengths of the finished paper as compared to the homopolymers withsimultaneous substantial improvement in wet strengths.

Example 3 Similar determinations to those in Example 1 were carried outwith c'opolymers of percent by weight of TABLE 1.EFFECT OF COPOLYMERCOMPOSITION Dry Burst Increase Wet Burst Wet/Dry Breaking IncreaseTreatment Factor Over Control Factor Control Length Over Control(Percent) (Percent) (Meters) (Percent) Untreated checks"... 22. 9 1 3,758

acrylamide with 20 percent of Z-amihoethyl methacrylate hydrochloride.The copolymers were added to the indicated cellulosic pulp in the amountof 1 percent by weight of the dry fiber content. The copolymers employedvaried in molecular weights as evidenced by a range of viscosities offrom 5 27 to 23,450 centipoises for aqueous 25 percent by Weightsolutions thereof at a temperature of 25 C. This latter property isindicated below as the Viscosity Characteristic. The dry strengthmeasurements are summarized in the following table wherein the pulp typeis indicated as follows: H=unbleached hardwood soda pulp beaten to aCanadian Standard freeness of 390, and P=bleached chemical pulp havingahigh content of alpha-cellulose.

Viscosity Pulp Dry Burst Characteristic Type Factor (c'entipoises) 23,450 H 37. 5 15, 450 H .36. 4 4, 050 H 38. 4 23, 450 P 44. 4 15, 450 P43. 7 8, 620 P 41. 7

Example 4 To illustrate the advantage of employing the copolymer of theinvention in the substantial absence of filler and its tolerance of thepresence of alum, a copolymer of the invention was employed in thepreparation of a series of handsheets containing in addition to thepolymer portions of (1) alum and (2) alum and clay filler. The testsheets were prepared and tested in a manner similar to that of Example 1except that they were cured for 20' minutes at 100 C. Also, the pulpemployed was a bleached softwood kraft beaten to 430 millilitersCanadian Standard freeness. The percent loading of the variouspaper-making ingredients in the test handsheet based on the dry fibercon-tent of the sheet and the results of the test operations includingthe basis weight, dry burst and tensile factors corrected to a constantbasis weight of 50 pounds are reported for each run in the following 1 Acopolymer containing 80% by weight acrylamide and 20% by weight2-an1inoethyl methacrylate hydrochloride.

2 Lbs. per 500 sheets having dimensions of 25 x 40".

Example 5 In further determinations similar to Example 2 above, it wasshown that the cop-olymer of 80 percent by weight of acrylamide with 20percent by weight of Z-aminoethyl acrylate hydrochloride, when employedfor treating unbleached hardwood soda pulp in amounts of from 0.5 to 1.5percent by weight of the cellulose, resulted in increased strength ofthe paper produced therefromof from about 32 to 45 percent when measuredby the dry burst test.

Paper prepared as in the above examples was analyzed for nitrogencontent to determine the retention of the active polymer in the finishedpaper. It was found that the percent of the added polymer retained inthe finished paper varied from about 10 to 80 percent of the polymericagent added, depending upon such factors as the molecular weight of thepolymer, the proportions of aminoalkyl ester moieties in the particularpolymeric agent employed and the type and prior treatment of thecellulosic pulp. Particularly favorable retention was obtained with thecopolymers prepared from to parts by weight of acrylamide andcorrespondingly from 20 to .10 parts by weight of Z-aminoethylmethacrylate hydrochloride.

In a manner similar to that of the foregoing, comparable improvements inthe dry and wet strengths of paper products are accomplished bysubstituting for the polymeric additives in Example 2, interpolymersunder the invention of 2-aminoethyl acrylate hydrochloride andmethacrylamide, which interpolymers also contain small amounts of otheracrylic monomers such as N-methylolacrylamide, acrylonitrile,methacrylonitrile, ethyl ochydroxymethylolacrylate, methyl methacrylate,hydroxyethyl methacrylate,'hydroxypropyl methacrylate and hydroxyethylacrylate, said interpolymers containing no more than about 25 percent byweight of the latter optional acrylic moieties.

What is claimed is:

1. A method for preparing paper having improved strength propertieswhich consists essentiallyof the steps of forming an aqueous suspensionof cellulosic fibers, incorporating in such suspension an amount withinthe range from about 0.01 to about 3 percent by weight of the drycellulose fibers of a water-soluble polymeric agent containingintralinear units of the general formulae:

R lay l )CnH2u-NH2 wherein R and R are independently selected from thegroup consisting of methyl and hydrogen, n is an integer from 2 to 3inclusive and x and y are the numbers of the monomer units in thepolymeric agent, the ratio of x/y being within the range from about 50to about 1.5 and forming a web by wet laying said fibers.

2. A method according to claim 1 wherein the polymeric agent is selectedfrom the group consisting of copolymers containing in polymerized formfrom about 97 to about 45 percent by weight of acrylamide andcorrespondingly from about 3 to about 55 percent by weight ofZ-aminoethyl methacrylate and mineral acid salts of the foregoingcopolymers.

3. A method for preparing paper having improved strength propertieswhich comprises the steps of forming an aqueous suspension of cellulosicfibers substantially free of an inorganic filler, incorporating in suchsuspension an amount from about 0.01 to about 3 percent by weight of thedry cellulose fibers of a water-soluble polymeric agent containingintralinear units of the general formulae:

H I l Tiff J.

and

and

wherein R and R are independently selected from the group consisting ofmethyl and hydrogen, n is an integer from 2 to 3 inclusive and x and yare the numbers of the monomer units in the polymeric agent, the ratioof x/ y being within the range from about 50 to about 1.5 and forming aweb by wet laying said fibers.

4. A method according to claim 3 wherein the polymeric agent is selectedfrom the group consisting of copolymers containing in polymerized formfrom about 97 to about.

and

10 wherein R and R are independently selected from the group consistingof methyl and hydrogen, n is an integer from 2 to 3 inclusive and x andy are the numbers of the monomer units in the polymeric agent, the ratioof x/y being within the range from about to about 1.5 and forming a webby wet laying said fibers.

6. A paper product as in claim 5 wherein the polymeric agent is selectedfrom the group consisting of copolymers containing in polymerized formfrom about 97 to about 45 percent by weight of acrylamide andcorrespondingly from about 3 to about percent by weight of Z-aminoethylmethacrylate and mineral acid salts of the foregoing copolymers.

References Cited by the Examiner UNITED STATES PATENTS 2,838,397 6/1958Gruntfest et a1 162168 2,972,560 2/1961 Stilbert et al 162168 3,007,88711/1961 Essig 162168 3,084,093 4/1963 Humiston 162168 FOREIGN PATENTS477,265 9/1951 Canada.

DONALL H. SYLVESTER, Primary Examiner.

MORRIS O. WOLK, Examiner.

S. L. BASHORE, Assistant Examiner.

3. A METHOD FOR PREPARING PAPER HAVING IMPROVED STENGTH PROPERTIES WHICHCOMPRISES THE STEPS OF FORMING AN AQUEOUS SUSPENSSION OF CELLULOSICFIBERS SUBSTANTIALLY FREE OF AN INORGANIC FILLER, INCORPORATING IN SUCHSUSPENSION AN AMOUNT FROM ABOUT 0.01 TO ABOUT 3 PERCENT BY WEIGHT OF THEDRY CELLULOSE FIBERS OF A WATER-SOLUBLE POLYMERIC AGENT CONTAININGINTRALINEAR UNITS OF THE GENERAL FORMULA: